Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion

Many CuI complexes have luminescent triplet charge-transfer excited states with diverse applications in photophysics and photochemistry, but for isoelectronic ZnII compounds, this behavior is much less common, and they typically only show ligand-based fluorescence from singlet π–π* states. We report two closely related tetrahedral ZnII compounds, in which intersystem crossing occurs with appreciable quantum yields and leads to the population of triplet excited states with intraligand charge-transfer (ILCT) character. In addition to showing fluorescence from their initially excited 1ILCT states, these new compounds therefore undergo triplet–triplet energy transfer (TTET) from their 3ILCT states and consequently can act as sensitizers for photo-isomerization reactions and triplet–triplet annihilation upconversion from the blue to the ultraviolet spectral range. The photoactive 3ILCT state furthermore facilitates photoinduced electron transfer. Collectively, our findings demonstrate that mononuclear ZnII compounds with photophysical and photochemical properties reminiscent of well-known CuI complexes are accessible with suitable ligands and that they are potentially amenable to many different applications. Our insights seem relevant in the greater context of obtaining photoactive compounds based on abundant transition metals, complementing well-known precious-metal-based luminophores and photosensitizers.


MATERIALS AND METHODS
Chemicals were obtained from commercial suppliers in high purity and were used without further purification. Dry solvents were used as purchased from commercial suppliers or from an Innovative Technology PureSolv micro multi-unit solvent purification system.
Nuclear magnetic resonance (NMR) spectroscopy was performed using either a 400 MHz Bruker Avance III spectrometer or a 600 MHz Bruker Avance III spectrometer at 298 K. The latter instrument was equipped with a direct observe 5-mm BBFO smart probe. Chemical shifts  are given in ppm (parts per million) and referenced to CDCl3 (7.26 ppm in 1 H-NMR and 77. 16 ppm in 13 C-NMR spectroscopy), CD2Cl2 (5.30 ppm in 1 H-NMR and 53.84 ppm in 13 C-NMR spectroscopy) or DMSO-d6 (2.50 ppm in 1 H-NMR and 39.52 ppm in 13 C-NMR spectroscopy). 1 The multiplicity of the signals is described with the following abbreviations: s (singlet), d (doublet), t (triplet), q (quartet), p (pentet), m (multiplet) and combinations of these abbreviations. The coupling constants J are given in Hertz (Hz).
High resolution mass spectroscopy (HRMS) was performed by Dr. Michael Pfeffer on a maxis 4G QTOF EDI spectrometer from Bruker. MALDI-TOF-MS was performed on a Bruker microflex instrument operating in positive mode. The matrix (DCTB in CH2Cl2) was evaporated onto the sample plate and then the substrate was evaporated onto the matrix. Photocatalytic reactions were performed in NMR tubes with tube caps from VWR. The irradiation source was a SOLIS-415C high-power LED from ThorLabs with a 380 or 400 nm cutoff filter.
Photostabilities were measured using a SOLIS-415C high-power LED from ThorLabs with a 400 nm cutoff filter or a Roithner Lasertechnik GmbH 405 nm continuous wave laser with an output power of 526 mW.
All photophysical measurements were carried out at 293 K and the solutions were purged with argon (4.8, PanGas) for at least 5 minutes using screw cap cuvettes. Steady-state optical absorption and UV-Vis spectro-electrochemical measurements were recorded using a Cary 5000 S2 spectrophotometer (Varian). Steady-state luminescence spectra were measured using a Fluorolog-3-22 instrument from Horiba Jobin-Yvon. Transient absorption and time-resolved absorption and emission measurements were performed on a LP920-KS instrument from Edinburgh Instruments. The excitation source was a pulsed Quantel Brilliant b ND:YAG laser equipped with an optical parameter oscillator (OPO) from OPOTEK or a Nd:YAG laser (Quantel Q-smart 450 mJ, ca. 10 ns pulse width) with a beam expander (BE02-355 from Thorlabs). The transient absorption spectra were detected on an iCCD camera (Andor), while kinetics at a single wavelength were recorded with a photomultiplier tube. Fluorescence lifetimes were measured on a LifeSpec II spectrometer (time-correlated single photon counting technique) from Edinburgh Instruments using picosecond pulsed diode lasers for excitation at 405 nm. The ligands m-LH and p-LH were synthesized starting with a previously published condensation reaction of the respective brominated salicylic acid and 2-aminophenol in polyphosphoric acid, 2 and then following a known synthesis strategy as displayed in Scheme S1. 3 . The Buchwald-Hartwig coupling with bis(4-methoxyphenyl)amine was specifically optimized for the reaction S4 partner m-2. The final deprotection of the phenolic oxygen was per formed in CH2Cl2 using palladium on activated charcoal under a hydrogen atmosphere, which gave the ligands m-LH and p-LH in excellent yields. Complexation was achieved by refluxing two equivalents of m-LH or p-LH with Zn(OAc)2 in tetrahydrofuran (THF) or toluene overnight. After filtration, the 1 H-NMR spectra of the complexes exhibited broad and ill-defined signals. In addition to the anticipated  5-Bromo-2-hydroxybenzoic acid (3.00 g, 13.8 mmol, 1.0 eq.) and 2-aminophenol (1.51 g, 13.8 mmol, 1.0 eq.) were suspended in polyphosphoric acid (8.0 mL). The reaction mixture was stirred at 180 °C overnight before it was allowed to cool to RT and poured into ice -water. The precipitate was collected by filtration, washed with water and dried in vacuo overnight, yielding the product (m-1, 3.18 g, 11.0 mmol, 80%) as a pink solid, which was used for the next reaction without further purification. Analytical data matches the literature. 3

2-(2-(BENZYLOXY)-5-BROMOPHENYL)BENZO[D]OXAZOLE (M-2)
This reaction was adapted from a previously published procedure. 3 bromophenol (m-1, 1.51 g, 5.20 mmol, 1.0 eq.) and Cs2CO3 (1.00 g, 5.18 mmol, 1.0 eq.) were dissolved in MeCN (33 mL). Benzyl bromide (0.62 mL, 5.22 mmol, 1.0 eq.) was added and the reaction mixture was stirred at 80 °C for 2.5 h before it was allowed to cool to RT and f iltered. The filtrate was diluted with CH2Cl2. The MeCN/CH2Cl2 mixture was washed with an aq. solution of NaOH (0.2 M) and brine, and then dried over anhydrous Na2SO4. The crude product was purified by column chromatography (SiO2, CH2Cl2 : EtOH 50:1) yielding the product (m-2, 1.58 g, 4.16 mmol, 80%) as a pink solid. Analytical data matches the literature.        For both complexes only minor signals in the HR-ESI-MS spectra were observed (with the expected isotope pattern and main peak at m/z=937), while the major peaks corresponded to the ligands m-LH and p-LH, indicating that the complexes are unstable upon electrospray ionization. This reaction was adapted from a previously published procedure. 3 4-Bromo-2-hydroxybenzoic acid (3.00 g, 13.8 mmol, 1.0 eq.) and 2-aminophenol (1.51 g, 13.8 mmol, 1.0 eq.) were suspended in polyphosphoric acid (10 mL) and stirred at 180 °C for 18 h. The reaction mixture was allowed to cool to RT and poured into ice-cold water. The precipitate was collected by filtration, washed with water and dried in vacuo yielding the product (p-1, 3.05 g, 10.5 mmol, 76%) as a green solid, which was used for the next reaction without further purification. Analytical data matches the literature.

4-(BENZO[D]OXAZOL-2-YL)-3-(BENZYLOXY)-N,N-BIS(4-METHOXYPHENYL)ANILINE (P-3)
This reaction was adapted from a previously published procedure. 3  (0.12 mL, 1.0 M in toluene, 0.120 mmol, 0.11 eq.) was added. The reaction mixture was refluxed overnight before it was allowed to cool to RT. CH2Cl2 and H2O were added. The phases were separated and the aqueous phase was extracted with CH2Cl2. The combined organic phases were washed with brine and dried over NaSO4. The solvent was removed under reduced pressure and the resulting crude product was purified by column chromatography (SiO 2, CH2Cl2 : petroleum ether 5:1 → neat CH2Cl2) yielding the product (p-3, 284 mg, 0.537 mmol, 51%) as a yellow solid.   immediately after heating the crude product to 220 °C under vacuum overnight. This step was of key importance to obtain the mononuclear target complex (see comments below Scheme S1). For both complexes only minor signals in the HR-ESI-MS spectra were observed (with expected isotope pattern and main peak at m/z=937), while the major peaks correspond to the ligands m-HL and p-HL, indicating that the complexes are unstable upon electrospray ionization.

LUMINESCENCE AND INTERSYSTEM CROSSING QUANTUM YIELDS AND LIFETIMES
The measurements of the luminescence quantum yields were carried out using a relative method, where the integrals of the steady-state emission spectra of a reference compound and the sample were determined. Then the luminescence quantum yields were calculated using equation S1.
eq. S1 In equations S1, Φfluo is the luminescence quantum yield, I the integral of the emission, OD the optical density at the excitation wavelength, n the refractive index (from literature 5 ) of the solvent.
Parameters without any subscript stand for the data of the sample, whereas parameters containing the subscript "r" stand for the data obtained for a reference compound. Both the reference compound as well as the main sample were excited at the same wavelength, where the concentrations of two solutions were adjusted to the same optical density. The optical density at the excitation wavelength was kept between 0.1 and 0.05.
eq. S10 The  Table 1 and Table S3 are weighted averages calculated according to eq. S11.

CYCLIC VOLTAMMETRY
For solubility reasons, cyclic voltammetry of the free (protonated) ligands was performed in acetonitrile, whereas dichloromethane was used for the two Zn II complexes. The m-LH ligand shows on oxidation wave at 0.61 V vs SCE ( Figure S12), which we attribute to a triarylaminebased oxidation process. The respective potential value seems in line with typical one -electron oxidation potentials of comparable triarylamines. 12 An irreversible reduction wave is furthermore observed below -2.0 V vs SCE, but this is not of interest for the current study.

S28
In the cyclic voltammogram of the free p-LH ligand ( Figure S14), the primary triarylamine oxidation occurs at 0.80 V vs SCE, i. e., at substantially higher potential than in the free m-LH ligand.
Similarly, in the [Zn(p-L)2] complex, the analogous oxidation process occurs at 0.80 V vs SCE ( Figure S12). We tentatively attribute the higher triarylamine oxidation potential in the [Zn( p-L)2] and p-LH compounds to different extents of electronic coupling between the phenolic substituent

PHOTOISOMERIZATION OF TRANS-STILBENE
To estimate the yield of the cis-stilbene product in the reaction shown in Table 2, 1 H-NMR resonances attributable to the trans-stilbene starting material and the cis-stilbene product were compared. Specifically, the integrals of the multiplet in the 1 H-NMR spectrum at 7.37-7.30 (Itrans) due to trans-stilbene and the singlet at 6.47 (Icis, marked by the triangles in Figure S19 and Figure   S20) attributable to cis-stilbene were used.

= +
eq. S13 For the calculation of the ratio of cis-stilbene to trans-stilbene in Figure S22, ycis was divided by ytrans.

PHOTOISOMERIZATION OF METHYL TRANS-CINNAMATE
In 1 H-NMR spectroscopy, the integral of the signal of the internal standard Me3SiPh was normalized to 1. The integrals of the methyl group resonance of the methyl trans-cinnamate substrate (Itrans) at 3.27 ppm and the methyl cis-cinnamate product (Icis) at 3.11 ppm were used for Figure S26: and Table 2.

PHOTOSTABILITY
The change in photosensitizer concentration c as a function of irradiation time was calculated according to eq. S14.
eq. S14 In equation S14, I0 is the normalized luminescence intensity of the intact photosensitizer prior to any photo-degradation and c0 is the concentration of the photosensitizer at the beginning of the long-term irradiation experiment. c0 was determined based on the photosensitizer's absorption spectrum and the molar extinction coefficient ε at 405 nm using the Lambert-Beer law (eq. S15).
In eq. S15, d is the path length of the cuvette. The initial photosensitizer concentrations were  (Table S4), to ensure that equal amounts of laser light are initially absorbed. To make the individual photostability data sets more directly comparable to one another, the representation used in Figure   S33, with changes of concentrations (c) rather than absolute concentrations, seemed meaningful. 14 S44 as in eq. S16. We determined ϕdecomp from the point at which I/I0=0.9, i. e., at the point at the luminescence intensity (I) has decreased to 90% of its initial value (I0).
= eq. S16 The number of photons emitted by the (nlaser) was calculated according to eq. S17.
Because not all photons emitted by the laser are indeed absorbed by the sample, the transmittance of the latter needs to be taken into account. The transmittance is the percentage of photons passing through the solution at 405 nm and is defined in eq. S18, where A is the absorbance at the relevant wavelength.
= 10 − eq. S18 The number of photons absorbed by the solution (nabs) is therefore in the following relationship with the number of photons emitted by the laser (nlaser).
= 0.9 • • 0 eq. S19 In eq. S19, the volume V is 0.003 L.   to estimate the energy differences between the lowest singlet and triplet excited states (ΔEST).
Natural transition orbitals (NTO) with a contribution greater than 5% were of main interest.
Ultimately, only the first two transitions from S0 to S1 and from S0 to S2 were considered, as these are the only transitions to exhibit sizeable oscillator strengths (>0.005) and transition energies For the [Zn(p-L)2] complex, qualitatively analogous but quantitatively different results are obtained.